1. Field
Circuit devices and methods for forming circuit devices.
2. Background
The field effect transistor (FET) is a common element of an integrated circuit such as a multiprocessor or other circuit. The transistor typically includes a source and drain junction region formed in a semiconductor substrate and a gate electrode formed on a surface of the substrate. The gate length is generally the distance between the source and drain junction region. Within the substrate, the region of the substrate beneath the gate electrode and between the source and drain junctions is generally referred to as a channel with a channel length being the distance between the source and drain junctions.
As noted above, many transistor devices are formed in a semiconductor substrate. To improve the conductivity of the semiconductor material of the substrate, dopants are introduced (e.g., implanted) into the substrate. Representatively, an N-type transistor device may have source and drain region (and gate electrode) doped with an N-type dopant such as arsenic. The N-type junction regions are formed in a well that has previously been formed as a P-type conductivity. A suitable P-type dopant is boron.
A transistor device works generally in the following way. Carriers (e.g., electrons, holes) flow between source junction and drain junction by the establishment of contacts on the substrate to the source and drain junction. In order to establish the carrier flow, a sufficient voltage must be applied to the gate electrode to form an inversion layer of carriers in the channel. This minimum amount of voltage is generally referred to as a threshold voltage (Vt).
In general, when fabricating multiple transistors of similar size, it is desired that a performance characteristic like threshold voltage be similar between devices. In general, the threshold voltage tends to decrease in response to reduced gate length. Of course, performance is often dictated by a reduction in transistor size (e.g., faster switching, more devices on a chip, etc.) that dominates the objectives of the semiconductor processing industry. As gate electrode lengths approach dimensions less than 100 nanometers (nm), what is seen is that the threshold voltage drops off or decreases rapidly. Therefore, even a small change in the gate electrode length (e.g., a 10 nanometer difference from a targeted length), can significantly affect the threshold voltage.
Ideally, the threshold voltage should be constant over a range of gate lengths about a target gate length to account for manufacturing margins. To, in one aspect, promote a more constant threshold voltage over a range of acceptable gate lengths, locally implanted dopants (P-type in N-type metal oxide semiconductor FETS (NMOSFETS) and N-type dopants in P-type metal oxide semiconductor FETS (PMOSFETS) may be introduced under the gate edges. Such implants are referred to as “halo” implants. The implanted dopant tends to raise the doping concentration around the edges of the channel, thereby increasing the threshold voltage. One effect is to reduce the threshold voltage of the target size device while maintaining the threshold voltage of the worst case size device.
Typical halo implants for NMOSFETS include boron (e.g., boron fluoride (BF2)) and indium (In). Halo implants for PMOSFETS include arsenic, antimony, and phosphorous. With respect to NMOSFETS, indium is a particularly preferred dopant because the channel of indium forms a retrograde profile from the surface of the device. Such a concentration profile with respect to indium, tends to decrease the threshold voltage required to meet a given leakage current (Ioff) in the device relative to a boron dopant which does not have the same retrograde profile. One problem with indium is that indium achieves a state of solid solubility at a point below the concentration required to reach worst case leakage currents. Thus, to target small leakage currents (e.g., on the order of 40 nanoamps (na) at device sizes less than 100 nanometers (nm)), a halo implant of an indium species alone cannot reach such targets.